Radars for motor vehicles were introduced for more comfort-oriented driving assistance functions, such as for example the ACC (Adaptive Cruise Control) function for use on motorways, or ‘Stop and Go’ in urban driving conditions. They use millimetric waves, in particular the 76-81 GHz band.
On account of technological developments, present applications also target collision avoidance safety functions, and there is even a view to, in the relatively short term, achieving a completely autonomous vehicle, with the perception of surroundings being ensured through the combination of a certain number of sensors based on various technologies, in particular radar, video and infrared.
Due to its all-weather capabilities, radar remains an important sensor in this context, and its detection and discrimination capabilities must be broadened in order to guarantee the overall reliability of the system. With regard to collision avoidance, the radar sensor must in particular be capable of distinguishing between the fixed objects that it detects, those corresponding to highway infrastructure elements and those corresponding to stationary vehicles on the road that potentially constitute a collision risk. In this context, it is vital in particular that it does not generate false alarms, which may lead to a braking operation or an emergency avoidance manoeuvre, without an actual cause, in particular when the vehicle is moving at high speed. This requires enhanced sensitivity and discrimination capabilities that make it possible to assess the situation a long distance, typically greater than 200 m, ahead of the vehicle, with in particular the capability of estimating the height of the detected objects in order to differentiate between them.
One particularly tricky configuration is that for detecting bridge decks ahead of vehicles, which bridge decks must not be confused with stationary vehicles on traffic routes. Moreover, in order to allow integration in vehicles, the dimensions of the radar antenna must typically be smaller than 10 cm horizontally by 10 cm vertically. This condition limits the angular resolution from the outset to a value of the order of 2.5° for a radar operating at 76 GHz, which is not sufficient for estimating the height of fixed objects at long distance.
One technical problem to be solved is that of obtaining discrimination capabilities for elevation that are sufficient for processing complex situations, such as detecting bridges at long distance, in spite of the small dimensions of the antenna.
This problem is not currently solved. Some solutions involve providing new-generation radars with at least two reception channels in the vertical plane, for the purpose of obtaining a capability of measuring the height of the detected objects through an angular deviation processing operation. However, this technique does not exhibit discrimination that is sufficient to estimate the height of the obstacles at long distance, due to the constrictive dimensions of the antenna. Moreover, it is not very well suited to measuring the height of objects extending in the vertical plane, and the measurement may be disturbed by reflections on the ground. Adaptive high-resolution processing operations may be implemented to improve the natural separating power of the antenna, but these processing operations are not effective in the presence of reflections on the ground, on account of the coherence of the direct and reflected signals.